Alcohol is an addictive recreational drug that reduces anxiety at low doses and causes sedation at high doses (1). These effects are similar to those of drugs that enhance the function of gamma-amino butyric acid sub A. receptor, referred to herein as GABAA receptor, which gate the Cl− currents that mediate most inhibitory neurotransmission in the brain. Gamma-amino butyric acid (GABA) is a major inhibitory neurotransmitter in the central nervous system. It mediates fast synaptic inhibition by opening the chloride channel intrinsic to the GABA (A) receptor. Acutely high doses of alcohol potentiate GABA-gated currents (1) at both native (1) and recombinant GABAA receptors (42), and chronically alter GABAA receptor expression (34). Low doses of alcohol have not been shown to directly modulate recombinant GABAA receptors (1), although there is indirect evidence for such effects at native receptors (6, 7, 29, 76).
It has been suggested that discrepancies between the alcohol sensitivity of native and recombinant receptors may be due to their subunit composition (64). Therefore there is a need to investigate the effects of low concentrations of alcohol on different subtypes of GABAA receptors.
Recently, a novel subunit combination of the GABAA receptor, α4β2δ, was identified (67), which is potentiated by very low concentrations of ethanol (1-3 mM), assessed using two electrode voltage clamp procedures and heterologous expression in oocytes. However, higher concentrations of ethanol (>10 mM) were ineffective in this regard, resulting in an “inverted U” concentration-response relationship. Because other subtypes were unresponsive to this low concentration of ethanol, including α4β2γ2 subunits of the GABAA receptor, this suggests that α4βδ subunits of the GABAA receptor possess a unique sensitivity to ethanol (67, 76).
Ethanol is known to exert behavioral (34) and electrophysiological (58) effects at relatively low concentrations. Low concentrations of ethanol can exert both postsynaptic and presynaptic effects in hippocampus and amygdala (8, 58, 76). This presynaptic effect of ethanol is enhanced when GABAB receptors are blocked (76), suggesting an effect of the drug on transmitter release, while effects on GABAergic transmission in amygdala are mediated via CRF1 receptors (54). Behavioral studies indicate that GABAA receptor antagonists/negative modulators can decrease ethanol consumption (32, 40), and ethanol can substitute for other GABA-modulatory drugs in drug discrimination tasks (18). Despite this evidence at native receptors, ethanol modulation of most recombinant GABAA receptor subtypes (50, 27), NMDA receptors (43), and glycine receptors (50) require comparatively high concentrations of the drug. In fact, the anesthetic effect of ethanol requires identified residues on the M2 region of the GABAA receptor (50). In contrast, the only recombinant receptor thus far identified as responsive to low concentrations of ethanol is the α4βδ subunit combination of the GABAA receptor.
The α4βδ subunits of the GABAA receptor have very low expression in most areas of the central nervous system (53, 81). The α4βδ subunits of the GABAA receptor also exhibit a distinctive response to GABA agonists, producing a greater maximum current in response to 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP or gaboxadol), a GABA partial agonist, compared to the maximum current gated by GABA (6), suggesting that 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol is a full agonist at α4βδ subunits of the GABAA receptor. A similar increase in the ratio of maximum 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol response to maximum GABA response was noted in isolated hippocampal neurons after progesterone withdrawal (67, 25), consistent with increased expression of α4βδ subunits of the GABAA receptor at this time. Coexpression of α4 and δ subunits of the GABAA receptor was also determined directly in hippocampus using co-immunoprecipitation techniques (67).
Under these conditions of increased expression of α4βδ subunits of the GABAA receptor after progesterone withdrawal, GABA-gated current recorded from isolated hippocampal neurons was also potentiated by 1-3 mM, but not higher (>10 mM), concentrations of ethanol (67). Suppression of α4 expression prevented the response to 1-3 mM ethanol after P withdrawal, further suggesting that α4βδ subunits of the GABAA receptor possess a unique sensitivity to ethanol. Interestingly, ethanol withdrawal also increases expression of the α4 subunit of the GABAA receptor (44, 14) but decreases expression of the δ subunit of the GABAA receptor (7), an effect correlated with a decrease in response to ethanol.
Therefore, there is a need to understand the relationship between the α4βδ GABAA receptor and increased sensitivity to alcohol in order to provide a mechanism for identifying and treating members of the general population at increased risk for alcoholism and premenstrual anxiety, also known as premenstrual syndrome, herein designated PMS.